The present invention relates to an efficient process for simultaneously producing ethylene glycol and a carbonate ester as industrial materials, especially ethylene glycol important as a raw material for polyester resin and a carbonate ester such as dimethyl carbonate useful as a raw material for polycarbonate resin.
For production of ethylene glycol (occasionally referred to simply as xe2x80x9cEGxe2x80x9d hereinafter), a route via ethylene carbonate (occasionally referred to simply as xe2x80x9cECxe2x80x9d hereinafter) has the advantage of giving ethylene glycol highly selectively with little production of dimers and trimers such as diethylene glycol and triethylene glycol over direct hydration of ethylene oxide and is studied extensively. The route to ethylene glycol via ethylene carbonate comprises reaction between ethylene oxide and carbon dioxide and subsequent hydrolysis of the resulting ethylene carbonate.
Meanwhile, production of a carbonate ester by transesterifying ethylene carbonate and a hydroxyl group-containing compound such as methanol is also known well.
However, these production processes are usually carried out independently in independent facilities.
The reaction between ethylene oxide and carbon dioxide for production of ethylene carbonate used by these two processes as the common starting material is slow, and it is known that addition of water is preferred to accelerate the reaction. Because addition of water also induces hydrolysis of ethylene carbonate into ethylene glycol, the reaction product has to be purified before used as the starting material for production of a carbonate ester. However, it takes laborious treatment to completely separate azeotropic ethylene carbonate and ethylene glycol.
On the other hand, in the process of producing ethylene glycol, addition of water for preparation of ethylene glycol is not problematic. However, because carbon dioxide obtained, for example, by burning hydrocarbons such as heavy oil as a precursor of ethylene carbonate is not immobilized in the product and is released from the system after the hydrolysis, there is a problem of leakage of carbon dioxide into the environment proportional to production of ethylene glycol.
Furthermore, because the transesterification involved in production of a carbonate ester is an equilibratory reaction, the reaction solution after the reaction contains unreacted ethylene carbonate. Recovery of ethylene carbonate from the remainder left behind the separation of the resulting target carbonate ester for reuse requires a separation procedure involving distillation. However, due to the presence of ethylene glycol in the reaction solution, the above-mentioned azeotropic property also comes up as a problem.
The object of the present invention is to provide a process which simplifies these two processes which involve cumbersome operations and release plenty of carbon dioxide from the system when conducted separately and reduces leakage of carbon dioxide into the environment.
As a result of the investigations to solve the above-mentioned problems, the present inventor has found out that the above-mentioned problems can be solved by combining the two processes with intervention by a step for purification of ethylene carbonate and have accomplished the present invention.
Namely, the present invention provides a process for simultaneously producing ethylene glycol and a carbonate ester comprising the following steps (a) to (c) for production of ethylene glycol, the following steps (d) and (e) for production of a carbonate ester and the following step (f) for purification of ethylene carbonate, wherein the step (f) intervenes to combine production of ethylene glycol and production of the carbonate ester:
(a) an EC formation step of reacting ethylene oxide and carbon dioxide to form a reaction solution containing ethylene carbonate,
(b) a hydrolysis step of reacting the solution containing ethylene carbonate with water to give an aqueous solution containing ethylene glycol,
(c) an EG purification step of purifying and collecting ethylene glycol from an aqueous solution containing ethylene glycol obtained mainly in the step (b),
(d) a transesterification step of transesterifying ethylene carbonate and a hydroxyl group-containing compound to form the corresponding carbonate ester and ethylene glycol,
(e) a carbonate purification step of separating the carbonate ester from the solution obtained in the step (d), and
(f) an EC purification step of separating ethylene carbonate by distillation from at least part of the reaction solution obtained in the step (a) and the remainder left behind the separation of the carbonate ester in step (e), feeding the ethylene carbonate to the step (d) and feeding the remainder to the step (b).
According to another aspect of the present invention, the present invention also provides the above-mentioned process for simultaneously producing ethylene glycol and a carbonate ester wherein ethylene glycol containing ethylene carbonate is separated from the remainder left behind the separation of the carbonate ester in step (e) as the top fraction and fed to the step (b) without passing the step (f), and the remainder as the bottom fraction is returned to the step (d).
Further, according to another aspect of the present invention, the present invention also provides the above-mentioned process for simultaneously producing ethylene glycol and a carbonate ester wherein the hydroxyl group-containing compound is a monohydric or dihydric alcohol, especially methanol.
According to still another aspect of the present invention, the present invention provides the above-mentioned process for simultaneously producing ethylene glycol and a carbonate ester wherein the ethylene oxide feed to the step (a) is obtained by oxidation of ethylene, and the carbon dioxide feed is a by-product of the oxidation of ethylene.